Electric current batteries

The tendency of many electrochemical reactions to move toward complete formation of products presents a significant challenge in the case of corrosion, but it also provides opportunities for using these reactions positively. The most familiar example of this is the battery, a cell or series of cells that generates an electric current. Batteries are composed of many different materials and find many uses, but they all share one common property—they provide a means by which we harness the electrical work of a galvanic cell and use it productively. We will see, however, that batteries have something else in common—they are susceptible to corrosion. So even when we intend to put electrochemistry to work for us, we still need to think about corrosion. Let s develop some ideas about batteries in general as we consider some specific examples. 

Electricity interacts with matter because electrons are part of matter and form the chemical bonds. When electrons are transferred from one molecule to the other we call it a redox reaction. Since electric current is the movement of electrons, micro electric currents then exist in the solution where redox reactions take place. If all these micro-currents were made to flow in one direction we should be able to measure them as one macro electric current. Batteries (which are also called galvanic cells or voltaic piles ) are devices which do exactly this they produce electric current by making redox reactions take place at electrodes, i.e. at the metal solution boundary. The metal can be either the source or the sink for electrons. Thus electric current is made to flow from the metal into the solution or from the solution into the metal. Can one do the reverse Can one induce redox reactions by passing through the solution current from a source The answer is definitely yes . The instrument by which such changes are produced is an electrolytic cell . A simple cell can be constructed from two pieces of dissimilar metals dipping into a solution of some electrolyte in a beaker. The metal pieces are now the electrodes. This book is concerned with chemical reactions produced by electric current or electric current produced by chemical reactions at electrodes. It is concerned with redox reactions in cells. 

Jictive mass is the material which generates electrical current by means of a chemical reaction within the battery. 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

The current needed for cathodic protection by impressed current is supplied from rectifier units. In Germany, the public electricity supply grid is so extensive that the CP transformer-rectifier (T-R) can be connected to it in most cases. Solar cells, thermogenerators or, for low protection currents, batteries, are only used as a source of current in exceptional cases (e.g., in sparsely populated areas) where there is no public electricity supply. Figure 8-1 shows the construction of a cathodic impressed current protection station for a pipeline. Housing, design and circuitry of the rectifier are described in this chapter. Chapter 7 gives information on impressed current anodes. 

The chemical process that produces an electrical current from chemical energy is called an oxidation-reduction reaction. The oxidation-reduction reaction in a battery involves the loss of electrons by one compound (oxidation) and the gain of electrons (reduction) by another compound. Electrons are released from one part of the batteiy and the external circuit allows the electrons to flow from that part to another part of the batteiy. In any battery, current flows from the anode to the cathode. The anode is the electrode where positive current enters the device, which means it releases electrons to the external circuit. The cathode, or positive terminal of the battery, is where positive current leaves the device, which means this is where external electrons are taken from the external circuit. 

An important property of this or any electrical circuit is the rate that charge moves past a place in the circuit (e.g., out from or into a battery terminal). The electrical current (I) is defined to be the charge (Q) that flows, divided by the time (t) required for the flow I = Q/t. In S.I. units the current (I) is in amperes (A). 

Rather than have pipes warmed by water, some heating systems employ coils of wire or other configurations of metal that are warmed by electric currents. An electric current is a flow of electric charge that may be either positive, negative or a combination of positive and negative. Both positive and negative electric currents are involved in the battery of an... 

Before 1831, the usual way of producing an electric current was by chemical means in the electric battery. Each cell of a battery had two different metals, or one metal and one carbon, separated by an acidic liquid. All electrical research in the first third of the nineteenth century made use of such batteries, and many combinations of materials were expired. 

Italian physicist Allcssandro Volta creates the first continuous electrical current by making a battery out of silver and zinc strips placed in salty water. Prior to this discovery all man-made electrical sources came from static. 

Most smoke alarms (Figure 19.1, p. 517) use a radioactive species, typically americium-241. A tiny amount of this isotope is placed in a small ionization chamber decay of Am-241 ionizes air molecules within the chamber. Under the influence of a potential applied by a battery, these ions move across the chamber, producing an electric current. If smoke particles get into... 

Electrochemical cells are familiar—a flashlight operates on current drawn from electrochemical cells called dry cells, and automobiles are started with the aid of a battery, a set of electrochemical cells in tandem. The last time you changed the dry cells in a flashlight because the old ones were dead, did you wonder what had happened inside those cells Why does electric current flow from a new dry cell but not from one that has been used many hours We shall see that this is an important question in chemistry. By studying the chemical reactions that occur in an electrochemical cell we discover a basis for predicting whether equilibrium in a chemical reaction fa-... 

The electrical current needed to start an automobile engine is provided by a lead storage battery. This battery contains aqueous sulfuric acid in contact with two electrodes. One electrode is metallic lead, and the other is solid Pb02. Each electrode becomes coated with solid PbSOq as the battery operates. Determine the balanced half-reactions, the overall redox reaction, and the anode and cathode in this galvanic cell. 

This is an electrochemical stoichiometry problem, in which an amount of a chemical substance is consumed as electrical current flows. We use the seven-step strategy in summary form. The question asks how long the battery can continue to supply current. Current flows as long as there is lead(IV) oxide present to accept electrons, and the batteiy dies when all the lead(IV) oxide is consumed. We need to have a balanced half-reaction to provide the stoichiometric relationship between moles of electrons and moles of Pb02. 

A battery is a galvanic cell that generates electrical current to power a practical device. Batteries can be as small as the buttons that power cameras and hearing aids or large charge storage banks like those of electric automobiles. 

Schematic diagram and examples of alkaline dry cells. These batteries provide electrical current for many portable devices.

In a galvanic cell, a spontaneous chemical reaction generates an electrical current. It is also possible to use an electrical current to drive a nonspontaneous chemical reaction. The recharging of a dead battery uses an external electrical current to drive the batteiy reaction in the reverse, or uphill, direction. 

Two directions of current flow in galvanic cells are possible a spontaneous direction and an imposed direction. When the cell circuit is closed with the aid of electronic conductors, current will flow from the cell s positive electrode to its negative electrode in the external part of the circuit, and from the negative to the positive electrode within the cell (Fig. 2.2a). In this case the current arises from the cell s own voltage, and the cell acts as a chemical source of electric current or battery. But when a power source of higher voltage, connected so as to oppose the cell, is present in the external circuit, it will cause current to flow in the opposite direction (Fig. 2.2b), and the cell works as an electrolyzer. 

During the next decades after the appearance of the Volta pile and of different other versions of batteries, fundamental laws of electrodynamics and electromagnetism were formulated based on experiments carried out with electric current supplied by batteries Ampere s law of interaction between electrical currents (1820), Ohm s law of proportionality between current and voltage (1827), the laws of electromagnetic induction (Faraday, 1831), Joule s law of the thermal effect of electric current, and many others. 

The appearance of electrochemical batteries provided an impetus in research into practical applications of electric current. The first prototype of electric telegraph appeared in 1804. In 1838, Jacobi experimented with a battery-driven motorboat on the Neva River not far from St. Petersburg. These achievements led to rapid development of the theory and practice of electrical engineering, and the seventh decade of that century saw the appearance of a revolutionary new power source the electromagnetic generator (Werner von Siemens, 1866), which soon surpassed their predecessors in both electrical and economic parameters. 

Several questions must be addressed with respect to the simple examples just outlined. Despite their formal similarity, it is important to keep in mind that the first instability describes an isolated system, where charge is a controlled variable. In contrast, the EC as introduced becomes unstable only in contact with a potentiostat (battery), when V is fixed and charge can change. Thus, the collapse of an EC leads to infinite growth of /l, accompanied by electric current from a battery to the EC. 

---